1. Technical Field of The Invention
The present invention relates to a data storage device for storing data and a method for operating a data storage device.
2. Background Art
In the field of this invention techniques are known that use nanometer sharp tips for imaging and investigating the structure of materials down to the atomic scale. Such techniques include scanning tunneling microscopy (STM) and atomic force microscopy (AFM), as disclosed in U.S. Pat. No. 4,343,993 and EP 0 223 918 B1.
Based on the developments of the scanning tunneling microscopy and the atomic force microscopy, new storage concepts have been introduced over the past few years profiting from these technologies. Probes having a nanoscale tip are being introduced for modifying the topography and for scanning an appropriate storage medium. Data are written as sequences of bits represented by topographical marks, such as indentation marks and non-indentation marks. The tips comprise apexes with a radius in the lower nanometer range and the indentation marks have for example a diameter in the range of 30 to 40 nm. Hence, these data storage concepts promise ultra-high storage area density.
In STM a sharp tip is scanned in close proximity to the surface and voltage applied between the tip and the surface gives rise to a tunnel current that depends on the tip-surface separation. From a data-storage point of view, such a technique may be used to image or sense topographic changes on a flat medium that represent a stored information in logical “0”s and “1”s. In order to achieve reasonable stable current, the tip-sample separation must be maintained extremely small and fairly constant. In STM, the surface to be scanned needs to be of a conductive material.
In AFM, the sharp tip rests on one end of a soft spring cantilever. When the sharp tip is in close proximity to a surface inter atomic forces may be sensed, which result in bending of the spring cantilever.
A storage device for storing data based on the AFM principle is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes scan during the operation an assigned field of the storage medium in parallel. That way high data rates may be achieved. The storage medium comprises a thin polymethylmethacrylate (PMMA) layer. The tips are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. For that purpose, the probes comprise cantilevers which carry the sharp tips on their end sections. Bits are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes in the surface while they are moved across the surface. Indentation marks are formed on the polymer surface by thermomechanical recording. This is achieved by heating a respective probe with a current or voltage pulse during the contact mode in a way that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation on the layer having a nanoscale diameter.
Reading is also accomplished by a thermomechanical concept. The heater cantilever is supplied with an amount of electrical energy, which causes the probe to heat up to a temperature that is not high enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium, especially a substrate on the storage medium, changes when the probe is moving in an indentation as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence, also its resistance changes. This change of resistance is then measured and serves as the measuring signal. Reading and also writing the marks is accomplished by moving the probes relative to the storage medium in lines within a track and moving to the next track when the end of the respective line has been reached.
Applicants EP 1 385 161 A2 discloses a storage device and a method for scanning a storage medium. The storage medium is designed for storing data in the form of marks and is scanned by an array of probes for mark detecting purposes in a scanning mode. The storage medium has fields with each field to be scanned by an associated one of the probes. At least one of the fields comprises marks representing operational data for operating the scanning mode. Scanning parameters are computed from the operational data and the scanning mode is adjusted according to the scanning parameters. The marks representing operational data may represent information for adjusting a tracking position. For that purpose, special marks are formed in respective fields of the storage medium, which are located in different positions relative to a track center line. By scanning these marks, information of the actual position of the probes relative to the track center line can be derived and used for adjusting a tracking positioning. Other fields comprise marks located in a periodic manner along respective lines within tracks. By scanning these fields, timing or clocking information may be obtained, which is used for adjusting the frequency of reading, writing or erasing pulses applied to the probes. These clocking or tracking adjustments take effect for all of the fields and the respective allocated probes.
It is a challenge to provide a data storage device and a method for operating a data storage device with a high data density and a low data loss rate.